1. Field of the Invention
The invention relates to a method of selecting a free channel in a cellular mobile radio system in which each rising communications path from the mobile station is associated with a given descending communications path from the fixed stations so as to form a duplex channel providing communication between any one of the fixed stations and any one of the mobile stations. In accordance with such method, in order to seek a free duplex channel between a fixed station F and a mobile station M:
in fixed station F, the already engaged duplex channels in the rising path for which the received power is higher than a first given threshold are detected and the addresses of such engaged channels are transmitted over one of the channels of a signalling path formed of equidistant time intervals successively allocated periodically to all the fixed stations of a set of cells constituting the cellular system;
in mobile station M, the addresses are determined of the engaged duplex channels in the descending path for which the received power is higher than a second given threshold, and a free duplex channel is selected from among channels having a different address then the addresses of engaged channels.
The invention also relates to apparatus for implementing this method in the fixed and mobile stations.
2. Description of the Related Art
Such a free channel selection method for a cellular mobile radio system is described in French Pat. No. 2 556 532 filed on Dec. 9, 1983, in the applicant's name, corresponding to U.S. Pat. No. 4,638,479, issued Jan. 20, 1987. This method has the advantage of making it possible to allocate all the communication channels to each cell in the system, whereas, in conventional systems, each set of channels is distributed in a group of "motif" cells, with the "motif" repeated throughout the communications system.
However, the implementation of this known method may give rise to difficulties in certain types of mobile radio system with respect to the determination in a mobile station of the engaged channels in the descending communications path.
Such implementation is convenient only in systems of the time-share multiple-access type (abbreviated to TSMA) in which the communication channels forming the rising and descending paths are time-multiplexed time channels. In a TSMA system, a mobile station has a wide-band receiver sequentially receiving all the channels of the descending path so that, in it, it is easy to detect those channels which are engaged simply by measuring the level of the signal received in the time intervals corresponding to the various channels and by comparing these levels with a predetermined threshold.
In mobile radio systems of the frequency division multiple-access type (abbreviated to FDMA), the communication channels are frequency-multiplexed frequency channels and the receiver of a mobile station can receive only one channel at a time so that, in order to measure the level in all the channels of the descending path, the receiver's frequency must vary in steps in order successively to explore all the channels. In a 255-channel system, for instance, to be explored in 125 ms (the frame period of a comparable TSMA system), each frequency step must last about 0.5 ms. To take account of the fading phenomenon caused by multiple tracks, an average of the measurement of the levels in the channels over two or three exploration processes must be struck. The relatively long time (2 to 3 times 125 ms) needed to seek a free channel by this method causes to major drawbacks during the stand-by period, and is highly annoying if the channel has to be changed during a communication, if, for example, the channel in use becomes cluttered. Communication must, in fact, be suspended while a free channel is being sought. To avoid this drawback, it is possible to use an auxiliary receiver continuously exploring the channels to seek a second free one in anticipation of the time when the main receiver has to change channels, but this is a complex solution.
Finally, it should be noted that there is a growing tendency in practice to use mixed systems combining frequency and time-distributed multiple-access systems making it easier than with purely TSMA of FDMA to provide the fixed stations with a variable number or channels depending on the area (urban or rural) or on the time to take account of the gradual increase in the number of users.
It is thus possible to produce a mixed TSMA/FDMA variable capacity system by frequency-multiplexing a variable number of TMSA frames. A distinction may be made between TSMA wide-band/FDMA systems in which each TSMA frame occupies a greater band-width than the coherence band of a channel, of the order of 200 KHz, and narrow-band TSMA/FDMA systems in which each TSMA frame occupies a coherence band of a channel. In both cases the fixed stations may be fitted with a variable number of channels depending on the location and the time, with an increment equal to the number of communication channels contained in a TMSA frame. Finally, it is also possible to set up a variable-capacity system which is a variant of a mixed narrow-band TSMA/FDMA system and ordinated among the users so as to avoid mutual interference. In such a system, a communications channel uses a TSMA frame time interval with a carrier frequency according to a code determined by the law of the frequency jumps used. In what follows, this mixed system is referred to as the TSMA/CDMA/FDMA system, where CDMA signifies "code-division multiple-access".
Clearly, if the free channel seeking method described in the above French patent application No. 2 556 532 is to be applied to the mixed system referred to above, all of which comprise frequency-distribution multiple-access, the same difficulties arise as with systems of the pure FDMA systems.